Led retrofit lamp

ABSTRACT

The present invention relates to an LED lamp ( 1 ) adapted for operation with an alternating current. The LED retrofit lamp ( 1 ) comprises a LED unit ( 7, 7′, 7″, 7 ′″) and a compensation circuit with a controllable switching device ( 9, 9 ′), connected parallel to said LED unit ( 7, 7′, 7″, 7 ′″) to provide an alternate current path. A control unit ( 10, 10′, 10 ″) is adapted to control said switching device ( 9, 9 ′) in a compensation mode in which said switching device ( 9, 9 ′) is set to the conducting state for the duration of a shunt period in each half cycle of the alternating current to allow adapting the power/current of the inventive LED lamp ( 1 ), so that a versatile and optimized operation of the lamp ( 1 ) is possible.

The invention relates to the field of lighting and particularly to anLED retrofit lamp adapted for operation with an alternating current.

BACKGROUND OF THE INVENTION

Recently, lighting devices have been developed utilizing light emittingdiodes (LEDs) for a variety of lighting applications. Due to theincreasing use of LEDs for lighting applications, LED lamps aredeveloped to replace common incandescent or fluorescent lamps, i. e. forretrofit applications. In addition to an increase in the lifetime ofsuch LED lamps in comparison to common lamps and thus lower cost, LEDlamps typically contain less hazardous materials, so that recyclingprocesses of such lamps can be conducted more efficiently.

For the above mentioned retrofit applications, the LED lamp typically isadapted to fit the socket of the respective fixture to be retrofitted.Furthermore, since a maintenance of a lamp typically is conducted by auser, the LED lamp should be readily operational with any type ofsuitable fixture without the necessity of an elaborate modification ofthe fixture.

LEDs typically exhibit a higher luminous efficacy than common lightsources and thus draw less current from the power supply for a givenluminous flux. While this fact is advantageous for the present effortsfor energy conservation, difficulties might arise when retrofitting afixture which is designed for a nominal power. Depending on the circuitdesign of the fixture, the differing current draw may result insignificant electrical problems, such as overheating of the ballast oran inacceptably low power factor when operating a LED lamp with suchfixture.

Therefore, it is an object of the present invention to provide an LEDretrofit lamp for operation with an alternating current which may bemanufactured cost-efficiently and allows a versatile and optimizedoperation, in particular with common types of fixtures.

SUMMARY OF THE INVENTION

The object is solved by an LED lamp according to claim 1, anillumination system according to claim 16 and a method of operating anLED lamp according to claim 17. Dependent claims relate to preferredembodiments of the invention.

The basic idea of the invention is to provide an alternating current LEDretrofit lamp with at least one LED unit, which lamp may be operatedcorresponding to given electrical specifications, such as powerconsumption, current and/or luminous flux in accordance with theapplication, i.e. the specifications of a given power supply unit or alamp ballast of a fixture. The inventive LED lamp thus allows anoptimised operation without the necessity of a change in theconfiguration of e.g. said power supply unit or fixture. The LED lampaccording to the present invention is thus particularly advantageous forthe use in retrofit applications, i. e. for replacing a commonfluorescent or gas-discharge lamp, since no alteration is necessary inthe wiring of the fixture.

The inventive LED lamp is particularly advantageous when used connectedwith a lamp ballast having a power factor correction (PFC) circuit,since for such applications it is important to maintain the averagecurrent through the lamp in a defined range to obtain a satisfyingcompensation result and thus a high power factor. However, the inventivesetup advantageously allows to operate the lamp also with a lamp ballastwithout a PFC circuit, so that a user may use the lamp in a variety offixtures without detailed knowledge of the circuitry and type ofballast. Therefore, only one type of lamp is needed to retrofit a giventype of fixture, which renders the installation process simple and thelamp cost-efficient.

According to the invention, the LED lamp is adapted for operation withan alternating current and comprises at least one LED unit, acompensation circuit with a controllable switching device, connectedparallel to said LED unit to provide an alternative current path and acontrol unit, adapted to control said switching device in a compensationmode, in which said switching device is set to a conducting state forthe duration of a shunt period in each half cycle of said alternatingcurrent. Depending on the control, the inventive LED lamp thus allows toadapt the electrical specifications, such as current or powerconsumption of the LED lamp and/or the LED unit to a predefinedcompensation value.

The inventive lamp is adapted for operation with an alternating current,such as for example provided by a 50/60 Hz mains supply line via asuitable power supply unit, e.g. a ballast unit of a lamp fixture.

The LED lamp according to the invention comprises at least one LED unit,which in terms of the present invention may comprise any type of solidstate light source, such as an inorganic LED, organic LED or a solidstate laser, e. g. a laser diode.

For general lighting applications, the LED unit may preferably compriseat least one high-power LED, i. e. having a luminous flux of more than 1lm. Preferably, said high power LED provides a luminous flux of morethan 20 lm, most preferred more than 50 lm.

For retrofit applications, it is especially preferred that the totalflux of the LED unit is in the range of 300 lm-10,000 lm, whichcorresponds to a typical 5 W-80 W fluorescent tube lamp. Mostpreferably, the forward voltage of the LED unit is in the range of 50V-200 V, particularly 70 V-150 V and most preferred 95 V-120 V.

The LED unit may certainly comprise further electric or electroniccomponents, such as a driver unit, e.g. to set the brightness and/orcolor, rectifying circuitry, a smoothing stage, a filter capacitorand/or a discharging protection diode. The LED unit may comprise morethan one LED, for example in applications where color-control of theemitted light is desired, e.g. using RGB-LEDs, or to further increasethe luminous flux of the LED lamp. Furthermore, the LED lamp maycomprise more than one LED unit, e.g. connected parallel to saidcompensation circuit.

The compensation circuit may be of any suitable type to provide analternative current path, i. e. an electrical connection parallel tosaid LED unit, e.g. allowing to at least temporarily bypass the LEDunit. The compensation circuit comprises at least a controllableswitching device, so that the circuit may be switched or controlled atleast to a conducting state and a non-conducting state.

In said conducting state, the compensation circuit provides analternative current path, so that during operation, an operating currentof the LED lamp is directed at least partly through the compensationcircuit. In the non-conducting state, the compensation circuit shows ahigh resistance, so that no substantial current passes through thecircuit, i.e. the residual current should preferably be less than 10 mA.Most preferably, the compensation circuit is open in the non-conductingstate, so that the full current is available to drive the at least oneLED unit.

The switching device may be of any suitable type to be recurrentlycontrolled to the conducting and non-conducting state. As will bediscussed in the following, at least one of the states can be set by thecontrol unit. The switching device should in addition be adapted to theelectrical specifications of the application in terms of maximal voltageand current, but also regarding switching frequency, i.e. to be set tothe conducting state in each half cycle of the alternating current. Theswitching device may preferably comprise a tyristor, a triac or anysuitable type of transistor, such as a MOSFET or bipolar transistor toset the state of the compensation circuit.

Beside the switching device, the compensation circuit may comprisefurther electric or electronic devices, e.g. a current-limiting device,such as a resistor or a reactive element, depending on the application.

Preferably however, the resistance of the compensation circuit is lowerthan the resistance of the LED unit, so that in the conducting state,the voltage across the compensation circuit and thus the LED unit islower than in the non-conducting state. Most preferably, the voltageacross the compensation circuit in the conducting state is less than 2V.Especially preferred, the compensation circuit is a low-resistancecircuit, i.e. having a maximum resistance of 20 Ohm.

According to the invention, the compensation circuit is connectedparallel to said LED unit. The LED lamp may certainly comprise furthercomponents, such as a housing, one or more lamp sockets adapted to therespective type of fixture, a smoothing stage, a flicker filter circuitand/or further control circuitry, e.g. to set the color of the emittedlight in case of a RGB LED unit. The parallel circuit arrangement of thecompensation circuit and the LED unit may be connected directly to saidlamp socket or via further electric components. Preferably, the LED lampcomprises rectifying circuitry, connected in series between the lampsocket and said parallel circuit arrangement of compensation circuit andLED unit for providing a direct current to drive said LED unit.Alternatively, it is also possible that said rectifying circuitry isformed integrally with said LED unit, as discussed above.

The LED lamp may be adapted to be connected to a PL-type fluorescentlamp fixture. However, according to a preferred embodiment of theinvention, the LED lamp comprises at least a first and second lamp cap.The lamp caps should be adapted to provide an electrical connection ofthe LED unit and the compensation circuit with the fixture respectiveand thus with power. The lamp caps may thus for example be provided witha corresponding contact element, such as a bi-pin base. For example, thelamp caps may have the electrical and/or mechanical properties of a T5or T8-fluorescent lamp. Preferably, the LED lamp is a LED tube lamp,such as a linear tube lamp. Most preferably, the LED lamp is adouble-capped tube lamp, e.g. having a first and second lamp cap,arranged on opposing ends of a housing.

The LED retrofit lamp according to the invention further comprises acontrol unit, adapted to control said switching device in a compensationmode. In this mode, the switching device is set to the conducting statefor the duration of said shunt period in each half cycle of saidalternating current, which e.g. may be supplied by a 50 Hz or 60 Hz lampballast.

The compensation circuit is thus recurrently activated, so that duringoperation the circuit provides an alternative current path to the LEDunit in each half cycle of the alternating current and thus draws adefined current from the ballast or power supply.

The control unit according to the invention may be of any suitable typeto enable a control of the switching device in the compensation mode asdescribed above. The control unit may therefore comprise discrete and/orintegrated electric or electronic components, a microprocessor and/or acomputing unit, e.g. with a suitable programming. Preferably, thecontrol unit is integrated with the switching device to provide a mostcompact setup.

Depending on the respective application, various control strategies maybe applied. For example, the control unit may be configured to controlthe power consumption of the LED unit to a predefined compensationvalue, e.g. substantially corresponding (+−10%) to the luminous flux ofa fluorescent lamp to be replaced. Alternatively or additionally, thecontrol unit may be configured to control the overall power consumptionof and/or the current through the lamp to a predefined compensationvalue for operation with a ballast with a PFC circuit.

As discussed above, the inventive LED lamp is particularly advantageouswhen used with a power supply or ballast having a PFC circuit since insuch cases it is important to maintain the current in a defined range toobtain the desired compensation of reactive power. In particular in caseof a parallel compensated ballast it is important to maintain a givencurrent through the lamp, since the current through the parallelcapacitance typically is fixed. Thus, the power factor of the overallarrangement mainly depends on the current through the series inductanceand the LED lamp.

Therefore, in particular in case of such power supply or ballast, thecontrol unit is preferably configured to control the current through thelamp to substantially correspond (+−10%) to the nominal current of thepower supply or ballast, e.g. the fluorescent lamp to be replaced.

The inventive LED lamp is advantageously further compatible with a powersupply without a PFC circuit, because the control of the shunt periodallows a flexible control. In addition, the inventive LED lamp mainly isa resistive load and advantageously does not cause a substantial phaseshift in the power supply, i.e. the alternating current and voltage. TheLED lamp may thus be used with a variety of power supply circuits andrespective fixtures, which renders the LED lamp particularlyadvantageous for retrofit applications.

According to a preferred embodiment, the control unit is configured toadapt the power consumption of the LED unit to a predefined compensationvalue. Since, as discussed above, during the conducting state, thecompensation circuit provides an alternative current path, the voltageacross the LED unit is accordingly reduced. Thus, the power consumptionof the LED unit may be set by the control unit by variation of theduration of the shunt period, so that the power consumption may easilybe set to the predefined compensation value.

The compensation value may be a fixed set-point value, e. g. factory setby the design of the control unit. Alternatively, the predefinedcompensation value may be variable, e.g. to be set by an installer usinga corresponding user interface and stored in a memory device, connectedwith the control unit. Thus, the installer may set the power consumptionof the lamp easily according to the respective application. Certainly,the term predefined compensation value may refer to a range, i.e. aminimum and maximum compensation value.

In the conducting state, the compensation circuit draws a certaincurrent, e.g. from the power supply or ballast. Therefore, acurrent-limiting device may be provided in series to the parallelcircuit arrangement of LED unit and compensation circuit to avoid thatthe current in the compensation circuit exceeds a safe level.

Alternatively or additionally, the LED lamp may be adapted for operationwith a reactive lamp ballast, such as e.g. a magnetic ballast unit of atypical fluorescent lamp fixture. Here, at least one reactive element,e.g. an inductance and/or capacitance, is connected in series to thelamp and thus limits the maximal current through the lamp. Thus, thecontrol unit may provide control of the power consumption of the LEDunit to the predefined compensation value without the need foradditional current-limiting devices in the lamp. According to thepresent embodiment, it is thus possible to adapt the power consumptionof the LED unit by corresponding control, while simultaneously a currentpath through the lamp is provided even during said shunt period, so thatthe current through the lamp can be maintained according to the nominalcurrent of the respective ballast, e.g. the nominal current of thefluorescent lamp to be replaced.

Certainly, the power supply or ballast may comprise more than onereactive element. For example and in a typical parallel compensatedfluorescent lamp ballast, the series inductance is compensated by aparallel PFC circuit having a suitable capacitance.

As discussed above, the control unit in said compensation mode isadapted to set the switching device to the conducting state for theduration of the shunt period in each half cycle of said alternatingcurrent. In particular in the case of a reactive lamp ballast, theswitching device is preferably set to the conducting state during areactive phase of said alternating current.

When using the lamp in combination with a reactive ballast or powersupply, the series reactive element causes a phase shift of the supplycurrent to the voltage. Thus, the supply provides power with effectiveand reactive phases. In the present context, the term “reactive phase”refers to an interval, where the product of voltage and current,supplied to the ballast, is negative, so that no effective or real poweris delivered to the load, i.e. the setup of ballast and LED lamp.According the present embodiment, the switching device is controlled tothe conducting state during a reactive phase of said alternatingcurrent. Since effective power is transferred from e.g. the mains gridto the ballast during an active phase, it is thus advantageouslypossible to decrease the power dissipation of the ballast, so that thepresent embodiment provides reduced heat generation in the ballast andlamp; resulting in correspondingly reduced loss.

According to a further preferred embodiment of the invention, theswitching device is controlled so that a shunt begin time or shunt endtime of said shunt period corresponds to a zero-crossing of saidalternating current.

In the context of the present invention, the terms “shunt begin time”and “shunt end time” refer to the moment of the change of state of theswitching device from the non-conducting to the conducting state andfrom the conducting to the non-conducting state, respectively, i.e. thebegin and end timing of the shunt period in the half cycle of thealternating current. The term “zero-crossing” refers to a moment ofsubstantially no current flow in each half cycle, i.e. when thealternating current approaches a zero-point, e.g. within an interval of+−1 ms prior or subsequent to the zero-crossing of the alternatingcurrent.

The present embodiment is particularly advantageous because at least onechange of state of the switching device in each half cycle is conductedat a moment, where the current is substantially zero, resulting in ahigher lifetime of the switching device and improved electromagneticcompability of the LED lamp. Furthermore, the present setup allows afurther simplified and thus more cost-efficient circuit setup.

For example, the switching device may comprise a self-latching switchingdevice, which is set to the non-conducting state when the current isbelow a defined holding current, e.g. near zero and thus may be referredto as zero-crossing detector. The self-latching switching device maye.g. comprise at least one tyristor or triac, which upon activationprovides a self-actuating reset when the alternating current approachessaid zero-crossing. Thus, the control and the corresponding setup of thecircuit is further simplified.

In the particular case of a connection of the lamp to a reactive powersupply having a series inductance, such as e.g. in an inductive lampballast, it is preferred that the switching device is controlled so thatsaid shunt end time corresponds to the zero-crossing of said alternatingcurrent to further decrease the power dissipation in the ballast. Thepower consumption and the duration of the shunt period is thencontrolled by the control unit, e.g. by a corresponding control of thetiming of the shunt begin time. Therefore, the present control is alsoreferred to as “leading edge control”.

In case of a capacatively series compensated ballast, i.e. a capacitiveballast, such as for example used in some typical “duo” fluorescent lampfixtures for one of the lamps in a capacitive branch of the circuit, itis preferred that the switching device is controlled so that said shuntbegin time corresponds to said zero-crossing of said alternatingcurrent.

Since here, the phase shift of the series capacitance causes the currentto lead the voltage, the control according to the present embodiment isadvantageous to decrease the power dissipation in said capacitiveballast. The present control is in the following also referred to as“trailing edge control”.

To set the above discussed preferred modes of operation, the controlunit may comprise a corresponding switch, so that an installer may setthe control mode of the switching device either to the leading edgecontrol in case of an inductive ballast or to the trailing edge controlmode in case of a capacitive ballast. Alternatively or additionally, thecontrol unit may preferably be adapted to operate in one or moredetection modes to automatically determine the most suitable controlmethod, which is in the following discussed with reference to a furtherpreferred embodiment of the invention.

According to a preferred embodiment, a voltage control circuit isconnected parallel to the LED unit and the compensation circuit to adaptthe forward voltage of the LED unit, e.g. in dependence of the currentthrough the LED unit. The voltage control circuit may e.g. provide areduction of the overall forward voltage of the LED unit by acontrollable shunting of a part of the LEDs. Thus, it is possible toprovide a further enhanced control of the power consumption of the LEDunit by a corresponding reduction of the forward voltage. The voltagecontrol circuit e.g. comprises a suitable switch to activate a furthercircuit, shunting at least one of the LEDs of the LED unit, butproviding that at least one of the LEDs is still connected with power.The switch may be operated in accordance with a given current levelreached.

Preferably, the control unit may comprise a detector to determine azero-crossing of said alternating current. The present embodimentadvantageously provides a more flexible control of the timing and inparticular the phasing/position of the shunt period in each half cycleof the alternating current.

The detector may be of any suitable type to determine said zero-crossingof the current. For example, the control unit may comprise amicroprocessor unit together with a suitable current detector, connectedwith the parallel circuit arrangement of LED unit and compensationcircuit. The control unit may then control said switching device andthus the shunt begin and/or shunt end time, e.g. according to thedesired duration of the shunt period and the respective type of ballast,as discussed above. The detector may for example comprise a currentmeasurement circuit for determining the current through the lamp, e.g.the parallel circuit of compensation circuit and LED unit.

Alternatively or additionally and particularly in case of the abovementioned self-latching switching device, the control unit preferablycomprises a threshold device, connected to said switching device. Thethreshold device may for example comprise a suitable type of DIAC, UJT(programmable unijunction transistor) or a comparator circuit having asuitable reference voltage. In case of a voltage driven thresholddevice, such as a DIAC, a driving circuit may be arranged to provide avoltage to the threshold device, which voltage is in a defined relationto the alternating current. Said driving circuit may additionallyprovide a delay period and/or comprise a voltage averaging stage. Thedriving circuit may e.g. be a RC-circuit, connected with said thresholddevice.

In case the threshold device is used in connection with saidself-latching switching device, the threshold device may e.g. be used totrigger said switching device to the conducting state according to thepredefined relation to said zero-crossing. The self-latching switchingdevice is then reset upon the next zero-crossing and the procedure isrepeated in the subsequent half cycle of the alternating current.

According to a further preferred embodiment of the invention, thecontrol unit is adapted to control the shunt begin time of said shuntperiod, so that the switching device is set to the conducting stateafter a first delay period after a zero-crossing of said alternatingcurrent.

The embodiment allows to easily set the shunt begin time in relation ofthe zero-crossing of the alternating current, which enables a flexiblecontrol of the shunt period and thus the power consumption of the LEDunit.

In a further preferred embodiment, the control unit is further adaptedto control the shunt end time of said shunt period, so that saidswitching device is set to a non-conducting state after a second delayperiod after a shunt begin time of said shunt period. The presentembodiment exhibits the advantage of a most flexible control of theshunt period and thus the power consumption/current of the lamp and/orLED unit, while still providing a rather simple setup. For example, itis possible to control the shunt period so that both, said shunt begintime and said shunt end time do not correspond to the zero-crossing ofthe alternating voltage, in the following referred to as “dual edgecontrol”.

The control unit may e.g. comprise at least one electronic timer toprovide the respective control signals after expiration of the firstand/or second delay period. Certainly, the timer may be providedintegrally with a microprocessor. Alternatively or additionally,RC-circuits may be used to provide said first and/or second delayperiod.

The first and/or second delay period may be fixed and for examplefactory set. Alternatively, the delay periods may be set by theinstaller using the above discussed used interface.

To further enhance the operation of the inventive LED lamp, the controlunit may preferably comprise feedback circuitry, also referred to as“feedback unit” or “feedback circuit”, to measure a current and/orvoltage of said LED lamp. The feedback circuitry may be of any suitabletype to measure the current and/or voltage of the LED lamp and thus e.g.the power consumption, so that during operation, the power consumptionof the LED lamp and/or LED unit may be adapted to the compensation valueaccording to the measurement of the actual consumption, i.e. in aclosed-loop operation. This embodiment is particularly advantageous,since the electrical characteristics of electronic components mightchange with temperature or due to aging. The feedback circuitry may e.g.be provided to measure the current and/or voltage of the parallelcircuit arrangement of LED unit and compensation unit. Alternatively oradditionally, the feedback circuitry may be provided to measure thecurrent and/or voltage of the LED unit and/or the compensation circuit.

The measured current and/or voltage may be used to set the duration ofthe shunt period, e.g. according to the control modes and timingprocedures discussed above. In particular, the control unit may comprisea PI- or PID-controller to allow a reliant and quick control of thepower to meet the predefined compensation value.

Preferably, the feedback circuitry is coupled to said control unit toset said first and/or second delay period according to said measuredcurrent and/or voltage.

According to a development of the invention, the control unit is furtheradapted to operate in a first detection mode, in which the switchingdevice is operated with a first set of timing control parameters, sothat a shunt end time of said shunt period corresponds to azero-crossing of said alternating current. Then, the current of said LEDlamp is determined. Following the measurement, the switching device isoperated with a second set of timing control parameters, so that saidshunt end time does not correspond to a zero-crossing of saidalternating current. The current of the LED lamp is again determined andin case the determined current according to said first set is less thansaid determined current according to said second set, the switchingdevice is operated with said first set of timing control parameters.

The operation in the above first detection mode allows to operate theLED lamp most efficiently, in particular in case the lamp is used inconnection with a reactive ballast unit, as described in the preceeding.As discussed above, it is preferable to operate the switching devicewhen using an inductive ballast with a leading edge control, where theshunt end time is set to correspond to the zero-crossing of the current.In case, e.g. an inductive ballast is driven using trailing edgecontrol, a relatively high ballast loss might result, which may causetemperature problems. Since it is typically not apparent to aninstaller, which type of lamp ballast is present in the fixture to beretrofit, the present embodiment advantageously allows to determine thespecific type of ballast to allow a most suitable control method.

The first detection mode may be initiated automatically upon connectionof the LED lamp to power. Alternatively or additionally, the firstdetection mode may be started by the installer, e.g. using the alreadydiscussed user interface.

Once the first detection mode is initiated, the control device operatesthe switching device with a first set of control parameters, so thatsaid shunt end time corresponds to a zero-crossing of said alternatingcurrent, i.e. a leading edge control method. The duration of the shuntperiod, i.e. the shunt start time may be chosen according to theapplication. For example, it may be possible to start with a defaultduration suitable to adapt the power consumption of the lamp to acompensation value suitable for most applications.

Preferably, the switching device is operated with said first set ofcontrol parameters for multiple half cycles of the alternating current,i.e. a stabilization period, so that the lamp power and thus the currentreaches a stable level.

The current of the LED lamp is then determined and e.g. stored in asuitable memory of the control unit, such as in a memory of amicrocontroller. The current may be determined by said feedbackcircuitry, as discussed above, which may be provided to measure thecurrent and/or voltage of the parallel circuit arrangement of LED unitand compensation unit. Alternatively or additionally, the feedbackcircuitry may be provided to measure the current of the LED unit and/orsaid compensation circuit.

The switching device is subsequently operated with a second set ofcontrol parameters with a shunt end timing, different from said firstset of timing control parameters. The second set of timing controlparameters may correspond to said trailing edge control, so that saidshunt begin time is set to correspond with the zero-crossing of thecurrent.

However, it is preferred that according to the second set of timingcontrol parameters, the shunt period is offset against the shunt periodaccording to the first set of control parameters by a predefineddetection offset. Most preferred, the detection offset is in the rangeof 1 ms-3 ms and especially preferred 2 ms for a mains frequency of 50Hz-60 Hz.

Especially preferred, the duration of the shunt period of said secondset of control parameters corresponds to the duration of said first set.

The current of said LED lamp is then again determined, preferably aftersaid stabilization period. The at least two determined currents arecompared, e.g. by said microcontroller. In case the power consumption ofthe lamp and thus—since the voltage is constant—the current according tosaid first set is lower than the current according to said second set,an inductive ballast is determined. Accordingly, the switching device isoperated with trailing edge control to provide a reduced ballast loss.The first detection mode then ends and the lamp may preferably beoperated in said compensation mode with the determined controlparameters, as discussed above.

In case the above mentioned comparison results in a reduced powerconsumption of said second set of control parameters, the switchingdevice is most preferably operated according to said trailing edgecontrol, i.e. so that said shunt begin time corresponds to azero-crossing of said alternating current.

According to a further preferred embodiment of the invention, anadditional controllable load switch is provided, arranged in series withsaid LED unit to at least temporarily disconnect said LED unit frompower.

The load switch may be of any suitable type to be controlled at least toa conductive and a non-conductive state, e.g. by the control unit over acorresponding control connection. The load switch e.g. may comprise oneor more transistors, such as bipolar transistors or MOSFETs. The loadswitch provides an operation of the lamp in the idle state, without theLED unit being connected with power, i.e. without being provided withsaid alternating current. The load switch is particularly useful in asecond detection mode, described in the following.

As discussed above, the load switch is arranged to control theconnection of said LED unit with power. Accordingly, the load switch mayin one example be arranged parallel to said compensation circuit,preferably integrated with said LED unit. To provide the above operationin the idle state, the switching device of said compensation circuit inthis case should be set non-conductive. Alternatively, the load switchmay be provided to control the connection of the overall parallelarrangement of the LED unit and the compensation circuit with power.

According to the above, the load switch allows to operate the lamp inthe idle state. Such operation may be particularly advantageous foroperation in the second detection mode.

According to a development of the invention, the control unit is adaptedto operate in the second detection mode, in which the load switch iscontrolled to disconnect the LED unit from power. Then the voltage atsaid LED lamp is determined and compared with a voltage threshold. Incase the determined voltage corresponds to said threshold, i.e. is equalto or higher than said threshold, the switching device is operated witha third set of timing control parameters. Otherwise, i.e. when thedetermined voltage is lower than said threshold, the switching device isoperated with a fourth set of timing control parameters, wherein theshunt period according to said third set of timing control parameters ineach half cycle of said alternating current does not substantiallyoverlap with the shunt period according to said fourth set of timingcontrol parameters.

The present embodiment accordingly provides an operation of saidswitching device according to a third and a fourth set of parameters independence of the voltage, present at the lamp without a load, i.e. insaid idle state. Certainly, the load switch should preferably be closedprior to the operation according to said third and fourth set of timingcontrol parameters to render the LED unit operational.

While the operation in said first detection mode, described in thepreceding, is useful for determining, whether the lamp is connected toan inductive or capacitive ballast in a 50 Hz mains grid system, theoperation in said second detection mode is particularly advantageouswhen the lamp is employed with a so-called “dual lamp rapid startballast”, typically used in 60 Hz mains grid systems. In contrast to theabove discussed ballast types, here two lamps are connected in serieswith each other and with an autotransformer. The ballast furthertypically includes a starter capacitor, connected parallel to one of thelamps and auxiliary cathode heater circuits, provided to igniteconnected fluorescent lamps.

The operation according to said third and fourth timing controlparameters allows an enhanced operation, when using two lamps accordingto the invention with a rapid start type of ballast, because the shuntperiods of the two lamps do not substantially overlap.

Due to the design of said rapid start type of ballast unwanted lossesmay occur due to excessive current flow in case the switching devices ofboth lamps would be set to the conductive state simultaneously.Accordingly, the power factor of the overall setup can be enhanced whenusing the inventive lamp with a dual lamp rapid start type of ballast.

In the present context, “no overlap” accordingly is understood toprovide that when two lamps are operated simultaneously, the positioningof the shunt period of the first lamp differs from the positioning ofthe shunt period of the second lamp in each half cycle of thealternating current, so that the switching devices of said two lamps arenot set to the conductive state simultaneously. However, a small overlap(+/−2 ms) is possible and comprised according to the presentexplanation, since the autotransformer in the present type of ballastdelays an excessive current flow even in case of such overlap.

To provide that one of the lamps in said rapid start type ballast isoperated with said third parameters and the respective other lamp isoperated with said fourth parameters, according to the presentembodiment, the voltage at the lamp in an idle state is determined.Since, as mentioned above, typical dual lamp rapid start ballastscomprise a starter capacitor parallel to one of the lamps, acorresponding voltage, higher than said threshold voltage, will bepresent at one of the lamps in said idle state, while said voltage isnot present at the respective other lamp. Thus the present embodimentallows to set one of the lamps according to said third set of parametersand the respective other lamp with the fourth set of parameters independence of the voltage in said idle state. A simultaneous shuntingthus can be advantageously avoided.

The shunt period and duration according to third and fourth set oftiming control parameters may be chosen according to the application, aslong as the shunt periods do not substantially overlap. For example, theoperation of the switching device according to said third set maycorrespond to trailing edge control, i.e. that the shunt begin time ofsaid shunt period corresponds to the zero-crossing of the alternatingcurrent. To provide no overlap, the control unit may further beconfigured so that the operation of the switching device according tosaid fourth set of parameters corresponds to leading edge control, i.e.that the shunt end time corresponds to the zero-crossing.

Preferably however, according to said fourth set of parameters, theoperation of said switching device corresponds to dual edge control toprovide an even further increased power factor. Most preferably, thetiming control parameters of said third and fourth set are chosen, sothat the according shunt periods are successive, i.e. that the shunt endtime according to the operation of one of the set of control parameterscorresponds (+−4 ms) to the shunt begin time of the operation accordingto the respective other set of parameters.

The voltage threshold according to the present embodiment should bechosen according to the application and preferably in dependence of thevoltage over the starting capacitor of the respectively used rapid starttype ballast. Preferably, the voltage threshold substantiallycorresponds to 175V (+−10%), which provides the operation discussedabove in typical rapid start ballasts. Most preferably, the voltagethreshold is an exclude range, providing increased stability. Forexample, the switching device may be operated with said third set, incase the determined voltage is higher than 190 V and according to saidfourth set when the voltage is lower than 160 V.

Although certainly the inventive lamp can be operated in an embodiment,where said control unit is adapted for operation according to the firstor the second detection mode, respectively, it is preferred that thelamp allows operating according to both of said first and seconddetection mode.

Here, an installer may set the respective detection mode, using theabove mentioned user interface. Alternatively or additionally and inview that the above type of dual lamp rapid start ballast is typicallyemployed in 60 Hz power grids while the inductive/capacitive types areused in 50 Hz power grids, the control unit may further preferablycomprise a frequency detector, so that the control unit operatesaccording to said first detection mode in case a 50 Hz (+−4 Hz)alternating current is determined and according to said second detectionmode in case a 60 Hz (+−4 Hz) alternating current is determined.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become apparent from the description of preferredembodiments, in which:

FIG. 1 shows an embodiment of an LED retrofit lamp according to theinvention in a schematic side view,

FIG. 2 a shows a schematic circuit diagram of an exemplary fixture foruse with the inventive LED lamp,

FIG. 2 b shows a schematic circuit diagram of a second exemplary lampfixture for use with the inventive LED lamp,

FIG. 3 shows the schematic circuit diagram of FIG. 2 a with a connectedLED lamp according to the embodiment of FIG. 1,

FIG. 4 shows a schematic diagram of an electric lamp circuit for an LEDretrofit lamp according to a first embodiment,

FIG. 5 a shows a timing diagram of the phasing of current and voltage inan inductive ballast,

FIG. 5 b shows a timing diagram of the operation of the embodimentaccording to FIG. 4,

FIG. 6 shows a schematic diagram of an electric lamp circuit accordingto a second embodiment,

FIG. 7 shows a timing diagram of the embodiment of FIG. 6,

FIG. 8 a shows a schematic diagram of an electric lamp circuit accordingto a third embodiment,

FIG. 8 b shows a schematic diagram of an electric lamp circuit accordingto a fourth embodiment,

FIG. 9 shows a schematic diagram of an electric lamp circuit accordingto a fifth embodiment,

FIG. 10 shows a schematic diagram of an electric lamp circuit accordingto a sixth embodiment,

FIG. 11 shows a further timing diagram of the operation of an LED lampaccording to the embodiment of FIG. 10,

FIG. 12 shows a schematic diagram of an electric lamp circuit accordingto a seventh embodiment,

FIG. 13 shows a further timing diagram of the phasing of current andvoltage in a capacitive ballast,

FIG. 14 shows a flow chart of the operation according to the embodimentof FIG. 12,

FIG. 15 shows a schematic diagram of an electric lamp circuit accordingto an eighth embodiment,

FIG. 16 shows a schematic circuit diagram of a third exemplary lampfixture for use with the inventive LED lamp,

FIG. 17 shows a further timing diagram of the operation of two LED lampsaccording to the embodiment of FIG. 15 and

FIG. 18 shows a flow chart of the operation according to the embodimentof FIG. 15.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows an embodiment of an LED retrofit lamp 1 according to theinvention in a schematic side view. The LED lamp 1 comprises a tube-likehousing 2, which extends along a longitudinal lamp axis 3. The housing 2is made from transparent plastic material, e.g. a polymethylmethacrylate(PMMA). On each of the longitudinal ends of the lamp 1, lamp caps 5 withcorresponding contact pins 6 are provided for connection to a typicallamp fixture, such as the fluorescent lamp fixtures 20, 20′, shown inthe schematic views of FIGS. 2 a and 2 b. Besides the electricalconnection, the lamp caps 5 also provide mechanical fixation and supportof the lamp 1 in the respective fixture 20, 20′. The LED lamp thus is aretrofit lamp, adapted for connection to a fixture 20, 20′ forfluorescent linear tube lamps. In the present case, the LED lamp 1 is areplacement for a typical TL-D 36 W fluorescent lamp, i.e. for aT8-tube, having a length of approximately 120 cm.

The contact pins 6 of the LED retrofit lamp 1 are connected with anelectric lamp circuit 4, which is explained in detail in the following.

FIG. 2 a shows a schematic circuit diagram of a typical embodiment of afluorescent lamp fixture 20. The fixture 20 comprises terminals 21 forconnection to a mains power supply 22, such as a 110/220V AC supplyline. For the connection of a lamp, such as the LED lamp 1, two sockets27 are provided, which according to the present example are of G13-type.The sockets 27 and thus an installed lamp 1 are connected with the mainspower supply 22 over lamp ballast 26. The lamp ballast 26 comprises aseries inductance 25, e.g. a suitable coil, which is usually employed tolimit the current in case a fluorescent lamp is installed in the fixture20 because of the negative impedance behavior of fluorescent lamps. Thelamp ballast 26 is in the following also referred to as “reactiveballast”.

Because the series inductance 25 causes a phase shift when operated withan alternating current, as can be seen from FIG. 5 a, a power factorcorrection (PFC) circuit 28 is connected parallel to the arrangement ofseries inductance and lamp 1, i.e. the ballast 20 thus is referred to asa parallel compensated “inductive or magnetic ballast”.

The PFC circuit 28 comprises a suitable capacitor 23 so that the phaseshift can be corrected, i.e. to obtain a sufficiently high power factor.As can be seen from FIG. 2 a, typical fluorescent lamp ballasts 26 areprovided with a parallel PFC circuit 28. Therefore, to provide a highpower factor, the current through the series inductance 25 and thus thelamp 1 needs to be matched to the design of the PFC circuit 28, i.e. thenominal load of the ballast 26 and thus the fixture 20.

The ballast 26 further comprises an auxiliary circuit 24, which isemployed to start a fluorescent lamp attached to the fixture 20. Whenusing the fixture 20 with the inventive LED lamp 1, the auxiliarycircuit is not necessary but may be left untouched, since the circuit 24does not hinder the operation of the LED lamp 1.

A second embodiment of a typical fluorescent lamp fixture 20′ is shownin FIG. 2 b in a schematic drawing. The lamp fixture 20′ is adapted tohold two lamps 1 and is accordingly equipped with a first pair ofsockets 27 and a second pair of corresponding sockets 27′. The firstpair of sockets 27 are connected with power over the series inductance25, as discussed above. The second pair of sockets 27′ are connectedwith power in series with a further inductance 25′ and a seriescapacitor 23′. According to the present example, the capacitor 23′ ischosen with a sufficiently high capacitance so that the inductive powerof both inductances 25, 25′ are compensated. Therefore, a dedicated PFCcircuit 28, as shown in FIG. 2 a, can be omitted with this circuitdesign.

The reactive ballast 26′ accordingly comprises an inductive branch,namely the circuit of inductance 25 and sockets 27, and a capacitivebranch, i.e. the circuit of the capacitor 23′, the inductance 25′ andsockets 27′. While in the inductive branch, the phasing corresponds toFIG. 5 a, i.e. the current 51 lags the voltage 52, in the capacitivebranch, the phasing corresponds to FIG. 13, i.e. here the current 51leads the voltage 52. Therefore, a lamp 1 connected to sockets 27 isprovided with an inductive ballast, while a further lamp 1 connected tosockets 27′ is provided with a capacitive ballast, so that the currentthrough both lamps 1 compensate for each other, provided that thecurrent and thus the power consumption in both lamps 1 substantiallycorresponds to each other. For reasons of clarity, correspondingauxiliary circuits 24 to start fluorescent lamps, connected to sockets27 and 27′ have been omitted in FIG. 2 b.

FIG. 3 shows the fixture 20 according to FIG. 2 a with a connected LEDlamp 1 according to FIG. 1. To not obscure the main aspects of thepresent explanation, all mere mechanical components, such as the housing2 of the LED lamp 1 or the sockets 27 of the fixture 20 have beenremoved in the schematic diagram of FIG. 3.

As can be seen from the figure, the lamp circuit 4 of the LED lamp 1 isconnected to the mains power supply 22 over the series inductance 25. Asdescribed above, the auxiliary circuit 24 is not used in the presentembodiment. The electric circuit 4 of the LED lamp 1 comprises arectifier 8, which converts the input alternating current 51, shown inFIG. 5 a, into an output current 53 of constant polarity, as shown inthe upper part of FIG. 5 b. The output of the rectifier is connected toan LED unit 7, which comprises in the present embodiment several high(or medium) power LEDs, connected in series.

The LED unit 7 is connected in parallel to a compensation circuit havinga controllable switching device 9. The controllable switching device 9is driven by a control unit 10 to a temporarily short-circuit the LEDunit 7 for the duration of a shunt period 57 in each half cycle of thealternating current 51 applied to the lamp 1. The lamp circuit 4 thusallows adapting the power consumption of the LED unit 7, independentfrom the forward voltage level of the LEDs.

The lamp 1 thus is particularly advantageous when employed incombination with a parallel compensated reactive ballast 26, as shown inFIG. 2 a, since here it is important that the current through the lamp 1meets the nominal load of the fixture 20 to achieve a high power factor.However, since the lamp 1 itself does not cause any substantial phaseshift, the lamp 1 can also be utilized with an uncompensated magneticballast (not shown), since no reactive power is added. The LED lamp 1thus is highly versatile.

A detailed embodiment of the lamp circuit 4 is shown in FIG. 4 in aschematic diagram. The circuit 4 is connected through the sockets 27(not shown in FIG. 4) of the lamp 1 via corresponding terminals 40. Theterminals are connected to the rectifier 8, which according to thepresent embodiment is a full-wave bridge rectifier comprising fourdiodes 41. A capacitor 43 is arranged to reduce electromagneticinterference. The input current 51, applied to the circuit 4 whenoperating the lamp 1 in e.g. fixture 20 is shown in the timing diagramof FIG. 5 a together with the line voltage 52 of the fixture 20 atterminals 21 for a full cycle of the current 51. The output current 53of the rectifier 8 is shown in the upper part of FIG. 5 b.

The output of the rectifier 8 is connected to the parallel circuit ofthe switching device 9, control unit 10 and LED unit 7. As shown, theLED unit 7 comprises several high-power LEDs 44. The total forwardvoltage of the LEDs 44 is 100V and thus approximately matches theoperating voltage of typical fluorescent lamps.

Furthermore, the LED unit 7 comprises a parallel connected smoothingcapacitor 45 and a diode 46 to avoid discharging of the capacitor 45when the switching device 9 short-circuits the LED unit 7.

According to the present embodiment, the switching device 9 is formed bya thyristor 47, i.e. a self-latching device, which gate terminal isconnected to a DIAC 48 of the control unit 10. The DIAC 48 serves as athreshold device to provide a defined timing of the begin of theshunt-period 57 with respect to the zero-crossing 55 of the current 51.The control unit further comprises a timing capacitor 49 and acorresponding resistor 50. The capacitor 49 and the resistor 50 form anRC-circuit to provide the DIAC 48 with a timing drive signal, so thatthe arrangement of DIAC 48, capacitor 49 and resistor 50 forms a “timingcircuit”. As discussed in the following, the drive signal follows thecurrent 51, but due to the characteristics of the RC-circuit, provides agiven delay period after each zero-crossing 55 until the thresholdvoltage of DIAC 48 is reached.

The operation of the lamp circuit 4 during use is hereinafter explainedwith reference to FIGS. 5 a and 5 b.

As already discussed, FIGS. 5 a and 5 b show timing diagrams of theinput alternating current 51 and the output current 53 at the output ofthe rectifier 8. Due to the reactive power of the series inductance 25,the line voltage 52 is phase shifted with respect to the input current51. The input current 51 accordingly provides reactive phases 54, asindicated in FIG. 5 a. During a reactive phase 54, no effective power istransferred from the power supply 22 to the series connection ofinductance 25 and lamp 1.

Beginning with the moment of a zero-crossing 55 of the current—or withreference to the output current 53 shown in FIG. 5 b a “zero-point”—thetiming capacitor 49 is charged over the resistor 50. As the voltageduring this phase is roughly constant at the level of the forwardvoltage of LEDs 44, charging of capacitor 49 is approximately a linearramp unit the threshold voltage of DIAC 48 is reached. When the voltagereaches the threshold voltage of DIAC 48, a current flows into the gateterminal of the thyristor 47, controlling the thyristor 47 to aconducting state. The LED unit 7 is accordingly short-circuited. Due tothe characteristics of thyristor 47, the device resets itself upon thenext zero-crossing 55 of the alternating current. The state of thethyristor 47 and the according timing of the shunt period 57, i.e. ashunt begin and shunt end time, is shown in the lower part of FIG. 5 b,where “0” represents the non-conducting state and “1” the conductingstate of the thyristor 47.

As will become apparent from FIGS. 5 a and 5 b, the control of thethyristor 47 using the DIAC 48 and the RC-circuit provides a delayperiod after each zero-crossing of the current 51. According to thepresent example, the shunt period 57 is set to correspond with thereactive phase 54. Therefore, during the conducting state, the powerdissipation in the ballast 26, 26′ does not increase substantially.However, the LED unit 7 is short-circuited during the shunt period 57,so that no voltage is applied to the LED unit 7. Because the current 51through the lamp 1 is limited by the series inductance 25, 25′, thepower consumption of the LED unit 7 is reduced.

This is particularly advantageous when using a common type of high-powerLEDs 44, as mentioned above. When using the shown series connection ofhigh-power LEDs 44 with a forward voltage of approx. 100V to meet thevoltage of a fluorescent lamp to be replaced, the resulting currentdrawn by the LEDs 44 is significantly higher than the current of atypical fluorescent lamp and thus the nominal current of a typicalballast 26, 26′ to provide a sufficiently high power factor.Accordingly, the present embodiment allows to set the power consumptionof the LED lamp 1 to the desired level.

FIG. 6 shows a second embodiment of lamp circuit 4′ of the LED lamp 1 ina schematic diagram. The present embodiment substantially corresponds tothe embodiment of FIG. 4, with the exception that the control unit 10′comprises a feedback circuit 60. While in the embodiment of FIG. 4 thedelay after the zero-crossing 55 until the thyristor 47 is set to theconducting state, is determined by arrangement of DIAC 48, resistor 50and timing capacitor 49, the feedback circuit 60 allows to adapt thedelay and thus the duration of the shunt period 57 according to theactual power consumption of the LEDs 44. Therefore, a variation in thepower consumption of the LEDs 44 due to aging or temperature can becompensated.

The feedback circuit 60 is connected to a current sensing resistor 61 ofthe LED unit 7′ to determine a voltage, corresponding to the presentcurrent through the LEDs 44. The thus obtained voltage is compared witha voltage reference 62, e.g. from a suitable voltage supply, todetermine a variation in the power consumption assuming a constantvoltage of LEDs 44. The feedback circuit 60 is further connected to theinput of DIAC 48. Depending on the determined variation, the feedbackcircuit 60 “bleeds” or draws a corresponding current from the timingcapacitor 59 to adapt the delay time and thus the duration of the shuntperiod 57, as shown in the lower part of timing diagram according toFIG. 7.

FIG. 8 a shows a third embodiment of lamp circuit 4″ in a schematicdiagram. The embodiment of FIG. 8 substantially corresponds to theembodiment of FIG. 6 with the exception of feedback unit 60′ and acorresponding low voltage supply circuit 62′.

The low voltage supply circuit 62′ comprises a resistor 81 and anarrangement of two zener diodes 82. A low voltage supply for thefeedback unit 60′ is coupled out via diode 83 and provides OP-amp 84with operating power. Additionally, the voltage reference signal isgenerated from the arrangement of shunt voltage reference 85, e.g.TL341, resistor 93 and resistors 86, 87, which form a voltage divider. Acapacitor 94 is provided as an energy buffer to smooth out the ripple inthe low voltage supply.

The OP-amp 84 is connected with capacitor 88 to form an error integratorfor the feedback control. The output of OP-amp 84 drives transistor 89,which draws a corresponding current from timing capacitor 49. Diode 95inhibits current flow from the transistor 89 to the timing capacitor 49.Resistor 96 assures that the capacitor 49 is not directly shunted.

The resistor 61 is used as current sensing resistor, as discussed above.The circuit of capacitors 90, 91 and resistor 92 form a low pass filterto extract a DC component of the voltage across resistor 61. The DCvoltage is then compared to the reference voltage at the positive inputof OP-amp 84. The error is then integrated by capacitor 88 and OP-amp 84to form the control signal for the transistor 89.

FIG. 8 b shows a fourth embodiment of the electric circuit 4′″ of an LEDlamp 1. The embodiment of FIG. 8 b substantially corresponds to theembodiment of FIG. 8 a, with the exception of a simplified feedback unit60″, which advantageously further reduces the overall cost of the LEDlamp 1.

Different from FIG. 8, no operational amplifier is used to control thetransistor 89 in the embodiment of FIG. 9. Instead an integrationcapacitor 97 is placed across the shunt voltage reference 85. Thefunctionality of an error integrator and voltage reference are accordingto the present embodiment both provided by shunt voltage reference 85and capacitor 97, which further simplifies the setup.

FIG. 9 shows a fifth embodiment of an electric circuit 4″″ of an LEDlamp 1. The embodiment of FIG. 9 corresponds substantially to theembodiment of fig.4, with the exception of LED unit 7″ and additionallya filter circuit 98 and a voltage control circuit 99. Filter circuit 98stabilizes the current for the LED unit 7″ to avoid visible flicker ofthe LEDs 44. Thus, the capacitance of capacitor 45 can be chosenrelatively small. Capacitor 45 reduces the “ripple” of the voltage,supplied to the LED unit 7″. A second filter stage is formed bydarlington transistor 131, resistors 133, 134 and capacitor 132.Resistor 133 and capacitor 132 form a RC highpass filter, dimensionedwith a relatively small capacitance and high resistance, thus enablingthe use of small and cheap components. The low ripple output voltage ofthis filter stage is amplified by transistor 131 to the full LED currentlevel. Dimensioning of resistor 134 sets the maximal load on the RCfilter output. In the phase where the input voltage is decreasing, theoutput of resistor 133 and capacitor 132 results that amplificationtransistor 131 can no longer operate. Current flows from capacitor 132through resistor 134 and transistor 131 to the LEDs 44. This reduces thevoltage on capacitor 132 to a value so that this phase is minimized andtransistor 131 is kept in an operation voltage range most of the time.An additional zener diode 130 prevents high voltage levels on thecapacitor 45 during startup. Since capacitor 132 is not charged in thefirst cycle after a startup, transistor 131 is not conductive and nocurrent flows to the LEDs 44, thus capacitor 45 is charged with fullmains voltage. For this time, diode 130 provides a second current pathand enables immediate current flow as soon as the voltage reaches thezener voltage of diode 130. During normal operation, the maximal voltageacross diode 130 is about the ripple voltage on capacitor 45 and thus isset to a non-conducting state.

The voltage control circuit 99 allows reducing the overall forwardvoltage of the LED unit 7″ by a controllable shunting of a part of theLEDs 44 by transistor 110. Thus, it is possible to provide a furtherenhanced control of the power consumption of the LED unit 7″ by acorresponding reduction of the forward voltage. Transistor 110 iscontrolled by the current through the LEDs 44. If the current throughthe LEDs 44 increases above a threshold, defined by resistor 111, e.g.if the voltage on resistor 111 increases above 0.7 V, transistor 110 isset conductive and short-circuits said part of the LEDs 44 through diode112. Simultaneously, transistor 113 is activated through resistors 114and 115. The corresponding current through resistor 116 keeps transistor110 in the conducting state, while the voltage across resistor 111 isreduced to zero, since the current now flows through transistor 110. Thelatched state of the circuit 99 prevails until the lamp 1 is switchedoff, so once a high current is detected the circuit 4′″ switches to an“high current” mode and is latched to this mode.

While the before-mentioned embodiments allow a relatively simple andthus highly cost-efficient circuit design, the control certainly islimited due to the thyristor 47, which links the end time of the shuntperiod 57 to the zero-crossing 55 of the alternating current 51.

FIG. 10 shows a sixth embodiment of a lamp circuit 4′″″, which allows ahighly flexible control. According to the present embodiment, theswitching device 9′ comprises a MOSFET 101 to at least temporarily shuntthe LED unit 7′. The gate terminal of the MOSFET 101 is connected to amicroprocessor 102 using a suitable gate driver unit 103. Themicroprocessor 102 comprises a feedback circuit, connected to currentdetectors 104 and 120, which may be e.g. simple sensing resistors asexplained with reference to FIG. 6. A temperature sensor 105 is furtherconnected to the microprocessor 102 to provide overheating protection.Furthermore, a voltage detector 106 senses the voltage of the output ofrectifier 8. A low voltage power supply 109 provides a suitable voltageto the gate driver unit 103, the microprocessor 102 and temperaturesensor 105. As discussed above, the embodiment according to FIG. 10allows a more flexible control of timing and the duration of the shuntperiod 57 so that the present embodiment advantageously allows a largercontrol range. The microprocessor 102 is provided with a suitableprogramming according to the application. For example, themicroprocessor 102 may be programmed with a first and second timer toset the shunt begin time according to a first delay period in responseto a zero-crossing 55 of the current 53. The duration of the shuntperiod and thus the amount of power reduction then is set by said secondtimer, which controls the shunt end time after to be set to a seconddelay period after the shunt period is started.

As shown in the timing diagram of FIG. 11, the present embodiment thusallows controlling the shunt period 57 so that the shunt begin time isset to the zero-crossing 55 of the current 51. Alternatively, it isfurther possible to adjust the shunt end time to the zero-crossing 55,as shown in FIG. 11 by the dotted lines. Further, it is possible tocontrol the shunt period 57, so that both, shunt begin and shunt endtime are different from zero-crossing 55, as shown in the lower part ofFIG. 17. The shunt period 57 thus can be freely positioned within saidhalf-cycle of current 51, also referred to as “dual edge control”, e.g.start in a first half-cycle and end in a subsequent half-cycle to allowa most flexible control. In addition, the present embodiment allows afurther improved operation in case of a capacitive ballast, as discussedin the following with reference to FIG. 12-14.

To allow setting the power consumption of the lamp 1, the microprocessor102 is programmed to control the MOSFET 101. The microprocessor 102determines the power of the LED lamp 1 in regular intervals by ameasurement of the voltage detector 106 and the current detector 104.The corresponding result is filtered, so that the average powerconsumption of the LED unit 7′ is determined. The microprocessor 102compares the average power consumption of the LED unit 7′ with thepredefined compensation value. According to the present embodiment, thepredefined compensation value is factory set in a memory (not shown),accessible to the microprocessor 102 according to the rating, i.e. thepower consumption of the LED unit 7′, which corresponds to the desiredflux of the LEDs 44. Based on the calculation, the microprocessor 102determines said first and second delay periods to set the first timerand the second timer.

The MOSFET 101 is then accordingly controlled in each half cycle of thecurrent 51. The microprocessor 102 determines the zero-crossing 55 ofthe input current 51 using current detector 120. Upon the detection of azero-crossing 55, the first timer is activated, which sets the MOSFET101 to the conducting state after the first delay period. Furthermore,the first timer triggers the second timer. After expiration of thesecond delay period, the second timer controls the MOSFET 101 to thenon-conducting state. The control cycle is then subsequently repeated ineach half cycle of current 51. Upon a detection of a change of the powerconsumption of the LED unit 7′ e.g. due to temperature or aging, thefirst and second delay periods are accordingly adjusted.

Since the microprocessor 102 is supplied with signals corresponding topresent voltage and current levels, it is possible to synchronize to themains frequency and to compensate for distortions of the zero-crossing55. The present embodiment using said microprocessor 102 further allowsto provide filtering and smoothing of the power, delivered to the LEDunit 7′. For example, the microprocessor 102 may alternatively oradditionally be programmed with a third timer, measuring the timeinterval between subsequent zero-crossings 55. By comparing the point intime when the real zero-crossing 55 occurs to the expected point intime, e.g. according to the previous timing of the zero-crossings 55, adistortion or disturbance is detected. Applying a fixed shunt timing toa unsymmetrical waveform might result in pulsation of the light outputand/or an amplification of the distortion.

The present embodiment therefore allows to determine a DC offset in themains supply or any other distortion resulting in a build up of a DCmagnetizing current, e.g. in a magnetic (uncompensated or parallelcompensated) ballast. In case of such distortion, the timing of theshunt period 57 is adapted with respect to the timing of thezero-crossing 55. The control unit 10′ thus is provided to detect andcompensate some distortion, or at least accept said distortion withoutfurther amplification.

In the event that the detected distortion is higher than a predefineddistortion limit, the operation of the lamp 1 is suspended, e.g. by aresettable fuse (not shown), so that the connection of the LED unit 7″to power is disrupted to prevent excessive DC input current.

FIG. 12 shows a seventh embodiment of a lamp circuit 4″″″ for the LEDlamp 1 in a further schematic diagram. The embodiment of FIG. 12corresponds to the embodiment of FIG. 10 with the exception of a lowpass filter 121 to provide the average current consumption of thecircuit 4″″″ to the microprocessor 102′. The current detector 120 isfurther adapted to detect the zero-crossing 55 and to provide acorresponding signal to the microprocessor 102′. As shown, the secondcurrent detector 120 may alternatively be provided to measure thecurrent through the compensation circuit and thus the switching device9′ and MOSFET 101. The overall current may then be determined by asimple addition of the current through the switching device 9′ and a LEDunit 7′.

According to the present embodiment, the microprocessor 102′ isprogrammed to operate in a first detection mode, e.g. upon connection ofthe LED lamp 1 with a fixture 20, 20′ and thus with the power supply 22.

In the first detection mode, it is determined whether the LED lamp 1 isconnected to an inductive ballast, as e.g. shown in FIG. 2 a, or to acapacitive ballast, e.g. the capacitive branch of fixture 20′, as shownin FIG. 2 b. Since when operating the lamp 1 with a capacitive ballast,the current 51 leads the line voltage 52, the timing of reactive phase54 and effective phase in each half cycle is opposite to the phasing inan inductive ballast, shown in FIG. 5 a.

FIG. 14 shows a flowchart of the operation of the circuit 4″″″ of FIG.12 during the first detection mode. As discussed above, the firstdetection mode starts with step 140 upon connection of the LED lamp 1with a fixture 20, 20′. The microprocessor 102′ controls the switchingdevice 9′ in step 141 according to a first set of control parameters, sothat the shunt end time corresponds to a zero-crossing 55 of thealternating current, i.e. in a leading edge control mode. The presentcontrol is maintained over a plurality of cycles of the alternatingcurrent 51, so that the lamp power is stabilizes. In step 142, thesecond current detector 120 determines the average lamp current and themicroprocessor 102′ correspondingly determines the average powerconsumption of the lamp 1.

Subsequently in step 143, the microprocessor 102′ operates the switchingdevice 9′ with a second set of control parameters. As can be seen fromthe right side of FIG. 14, the shunt period 57 according to theoperation of step 143 is offset with respect to the operation in step141, e.g. by 2 ms. After some cycles of the alternating current theaverage current is again determined using the second current detector120 in step 144.

The microprocessor 102′ then determines in step 145 whether thedetermined current of step 142 is less than the current determined instep 144. If this is the case, an inductive ballast is determined.Accordingly, the switching device 9′ is in step 146 controlled, so thatsaid end of the shunt period corresponds to the zero-crossing 55 of thealternating current, i.e. leading edge control. Thus, it is assured thatthe shunt period 57 is set to the reactive phase of current 51, so thatthe current in the inductor 25 does not substantially increase whenshort-circuiting the LED unit 7′. The control thus provides reducedballast loss.

On the other hand, if the current, measured in step 142 is higher thanthe current of step 144, a capacitive ballast is determined. Here, theswitching device 9′ is in step 147 controlled, so that the shunt begintime corresponds to the zero-crossing 55 of the alternating current,i.e. a trailing edge control mode. Accordingly, the shunt period 57 isagain set to the reactive phase of current 51 when operating the lamp 1with a capacitive ballast.

The first detection mode then ends and the switching device 9′ isoperated with the determined control mode. The duration of the shuntperiod 57 is controlled according to the measured power consumption ofthe LED unit 7′, as explained with reference to FIG. 10. According tothe embodiment discussed above, the current through the lamp isdetermined to select the timing control method corresponding to therespective ballast type. In case the current, measured in steps 142 and144 does not substantially differ from each other, a further criterionto select the appropriate control method is to determine the timing ofthe zero-crossing 55, as discussed above with reference to FIG. 10.Therefore, the microprocessor 102′ may additionally or alternatively beadapted to determine the point in time of subsequent zero-crossings 55in steps 142 and 144 to determine which set of control parametersaccording to steps 141 and 143 provides the least distortion in thealternating current 51 and then controls the switching device 9′accordingly.

FIG. 15 shows an eighth embodiment of a lamp circuit 4′″″″ of an LEDlamp 1 in a further schematic diagram. The embodiment of FIG. 15corresponds to the embodiment of FIG. 10 with the exception of anadditional MOSFET load switch 150, connected with the microprocessor102″ over a gate driver (not shown). The switch 150 allows to controlthe connection of the LED lamp 7′″ with power and allows to determinethe voltage in an idle state using the voltage detector 106. Certainlythe gate driver, although not shown, is connected with the low voltagepower supply 109 during operation.

According to the present embodiment, the microprocessor 102″ isprogrammed to operate in a second detection mode, e.g. upon connectionof the LED lamp 1 with the power supply 22. The operation according tosaid second detection mode is particularly advantageous when operatingthe lamp 1 with fixture 20″ according to FIG. 16, which comprises a duallamp rapid start ballast 26″, as typically used in 60 Hz mains gridsystems.

Fixture 20″ according to FIG. 16 is adapted to hold two lamps 1 and thuscorresponds in general to the setup of fixture 20′, shown in FIG. 2 b.However, as will become apparent from the figure, the correspondingsockets 27, 27′ are arranged, so that the lamps 1 are connected inseries with each other. In addition to the above mentioned seriesinductance 25 and the capacitor 23′, the rapid start type ballast 26″further comprises a starting capacitor 160, which in combination withautotransformer 161 and auxiliary cathode heater circuits 162 allowsigniting fluorescent lamps when attached to fixture 20″.

While in the embodiment according to FIG. 2 b the LED lamps 1 mayoperate independently from each other, due to the series connection ofthe LED lamps 1 according to the setup of fixture 20″ a simultaneousoperation of MOSFETs 101 of both installed lamps 1 and thus overlappingshunt periods 57 should be avoided to enhance the power factor of theoverall setup.

In the second detection mode, it is thus determined, whether the lamp 1is connected to sockets 27 (right side in FIG. 16) or to sockets 27′(left side) and to control the lamp either according to a third set oftiming control parameters or to a fourth set of timing controlparameters to avoid simultaneous operation of MOSFETs 101 of bothinstalled lamps 1 and thus simultaneous shunting.

FIG. 18 shows a flowchart of the operation of the circuit 4′″″″ duringthe second detection mode. The operation in the second detection modestarts in step 180 upon connection of the LED lamp 1 with fixture 20″,i.e. with the power supply 22. The microprocessor 102″ then controls theload switch 150 and the switching device 9′ to the open, non-conductivestate in step 181. Subsequently, microprocessor 102″ queries the voltagedetector 106 after some cycles of the alternating current in step 182 todetermine the idle voltage at the LED lamp 1 without the LED unit 7′″being connected with power and thus in said idle state. The load switch150 is then closed, i.e. brought into a conductive state in step 183.

The microprocessor 102″ determines in step 184, whether the idlevoltage, determined in step 182 is equal to or higher than a voltagethreshold of 175 Volt, set according to the present example of rapidstart ballast 26″. In case the idle voltage is equal to or higher thanthe voltage threshold, it is determined that the lamp 1 is connected tosockets 27 of the fixture 20″, i.e. the right hand side of FIG. 16.According to the present example, the switching device 9′ is in thiscase operated according said third set of parameters (step 186) so thatthe shunt begin time corresponds to the zero-crossing 55 of thealternating current, i.e. trailing edge control.

In the respective other case that the idle voltage is lower than thevoltage threshold, it is determined that the lamp 1 is connected tosockets 27′ of the fixture 20″ and thus the left hand side of FIG. 16.Here, the switching device 9′ is operated according to said fourth setof parameters (step 185) with dual edge control, i.e. both, shunt beginand shunt end time do not correspond to the zero-crossing 55. The seconddetection mode then ends and the switching device 9′ is operatedaccording to the determined set of control parameters.

When according to the above, two lamps 1 are correspondingly operatedaccording to the second detection mode in a dual lamp rapid start typefixture 10″ according to FIG. 16, it follows that one of the lamps 1 isoperated with trailing edge control and the respective other lamp isoperated with dual edge control. Accordingly and as can be seen from thetiming diagram of FIG. 17, the shunt period 57 a of said first lamp 1does not overlap with the shunt period 57 b of said second lamp 1 ineach half cycle of the operating current 51 or the corresponding outputcurrent 53 of rectifier 8, shown in FIG. 17. The present embodiment thusprovides a high power factor when the lamp 1 is operated in a rapidstart type ballast 26″.

Although the above operation in the first detection mode according toFIG. 14 and the second detection mode according to FIG. 18 have beendescribed separately for reasons of clarity, it is nevertheless possibleto operate the invention in an embodiment where both detection modes areemployed to obtain a highly versatile LED lamp 1.

Here, an installer may set the respective detection mode, using a userinterface or switch. Alternatively or additionally and in view that theabove type of dual lamp rapid start ballast is typically employed in 60Hz power grids while the inductive/capacitive types are used in 50 Hzpower grids, the control unit 10′ may further preferably comprise afrequency detector, so that the control unit 10′ and the microprocessor102″ operates according to said first detection mode in case a 50 Hzalternating current is determined and according to said second detectionmode in case a 60 Hz alternating current is determined.

The invention has been illustrated and described in detail in thedrawings and the foregoing description. Such illustration anddescription are to be considered illustrative or exemplary and notrestrictive; the invention is not limited to the disclosed embodiments.It may for example be possible to operate the invention according to anembodiment in which:

-   -   the LED unit 7, 7′, 7″, 7′″ only comprises a single LED 44,    -   the LED unit 7, 7′, 7″, 7′″ comprises an OLED or a laser diode        as light emitting element,    -   the switching device 9, 9′ and/or the control unit 10, 10′, 10″        are formed integrally with one of the lamp caps 5,    -   instead of rectifier 8, a rectification circuit is formed        integrally with the LED unit 7, 7′, 7″, 7′″,    -   the arrangement according to the embodiment of FIG. 9 is        combined with one of the embodiments of FIG. 8, 8 a or 8 b, i.e.        with a feedback circuit 60, 60′ or 60″,    -   in the embodiments of FIGS. 10, 12 and/or 15, the generation of        the control signal for MOSFET 101 is realized by an analog timer        circuit, controlled by microcontroller 102, 102′, 102″    -   instead of a MOSFET 101, a bipolar transistor, IGBT or different        type of controllable switch is used,    -   in the embodiments of FIGS. 10, 12 and/or 15 a user interface is        provided, so that the predefined compensation value may be set        by a user,    -   in the embodiment of FIG. 12, alternatively or additionally to a        determination of the current in steps 142 and 144, the power        consumption of the LED unit 7′ is determined and/or    -   in the embodiment of FIG. 15, a voltage threshold different from        175 V is chosen or the voltage threshold is an exclude range.

1. An LED retrofit lamp adapted for operation with an alternatingcurrent and a reactive lamp ballast, the lamp comprising a LED unit, acompensation circuit with a controllable switching device, connectedparallel to said LED unit to provide an alternative current path and acontrol unit, adapted to control said switching device in a compensationmode, in which said switching device is set to a conducting state forthe duration of a shunt period in each half cycle of said alternatingcurrent, wherein said control unit is configured to adapt the powerconsumption of the LED unit to a predefined compensation value; andwherein, when applied in combination with the reactive lamp ballast,said switching device is set to the conducting state during a reactivephase of said alternating current. 2-4. (canceled)
 5. The LED retrofitlamp according to claim 1, wherein said switching device is controlledso that a shunt begin time or shunt end time of said shunt periodcorresponds to a zero-crossing of said alternating current.
 6. The LEDretrofit lamp according to claim 1, wherein said switching device is aself-latching switching device.
 7. The LED retrofit lamp according toclaim 1, wherein said control unit comprises a detector to determine azero-crossing of said alternating current.
 8. The LED retrofit lampaccording to claim 1, wherein said control unit is adapted to control ashunt begin time of said shunt period, so that said switching device isset to the conducting state after a first delay period after azero-crossing of said alternating current.
 9. The LED retrofit accordingto claim 8, wherein said control unit is further adapted to control ashunt end time of said shunt period, so that said switching device isset to a non-conducting state after a second delay period after a shuntbegin time of said shunt period,
 10. The LED retrofit lamp according toclaim 8, wherein said control unit comprises feedback circuitry tomeasure a current and/or voltage of said LED lamp.
 11. The LED retrofitlamp according to claim 10, wherein said control unit is adapted tocontrol the duration of said shunt period according to said measuredcurrent and/or voltage.
 12. The LED retrofit lamp according to claim 10,wherein said control unit is further adapted to operate in a firstdetection mode, in which the switching device is operated with a firstset of timing control parameters, so that a shunt end time of said shuntperiod corresponds to a zero-crossing of said alternating current, thecurrent of said LED lamp is determined, the switching device is operatedwith at least a second set of timing control parameters, so that saidshunt end time does not correspond to a zero-crossing of saidalternating current, the current of said LED lamp is determined and incase the determined current according to said first set is less than thedetermined current according to said second set, the switching device isoperated with said first set of timing control parameters.
 13. The LEDretrofit lamp according to claim 10, further comprising a controllableload switch, arranged in series with said LED unit to at leasttemporarily disconnect said LED unit from power.
 14. The LED retrofitlamp according to claim 13, wherein said control unit is adapted tooperate in a second detection mode, in which the load switch iscontrolled to disconnect said LED unit from power, the voltage at saidLED lamp is determined and compared with a voltage threshold and in casethe determined voltage is equal to or higher than said voltagethreshold, the switching device is operated with at least a third set oftiming control parameters and in case the determined voltage is lowerthan said voltage threshold, the switching device is operated with atleast a fourth set of timing control parameters, wherein the shuntperiod according to said third set of timing control parameters in eachhalf cycle of said alternating current does not substantially overlapwith the shunt period according to said fourth set of timing controlparameters.
 15. The LED retrofit lamp according to claim 13, the controlunit further comprises a frequency detector, so that the control unitoperates according to said first detection mode in case a 50 Hzalternating current is determined and according to said second detectionmode in case a 60 Hz alternating current is determined. 16-17.(canceled)